The present invention relates to a driving system for a synchronous machine, and particularly, relates to a driving system suitable for an electric vehicle having a permanent magnet type synchronous machine.
In general, a hybrid driving system for an electric vehicle is composed of a generator driven by an engine, a battery, a motor for driving the vehicle by using the generator or the battery as the electric power source and respective driving circuits for the generator and the motor, in which the respective driving circuits have their own electric power converters. Two methods of driving a vehicle are known. One is a services type in which the engine, the generator and the motor are linked in series, and the other is a parallel type in which the engine, the generator and the motor are linked in parallel. In the series type, the vehicle is always driven by the motor. The electric power for driving the motor is supplied from the battery or the generator linked to the engine, and at the same time the battery is charged by the generator linked to the engine.
The prior art of the hybrid driving systems of this kind is disclosed, for example, in Japanese Patent Application Laid-Open No. 7-336809. The prior art is a series type hybrid driving system in which output power of the generator driven by the engine is supplied to charge the battery using a diode rectifying circuit.
On the other hand, irrespectively of whether the motor is an induction motor or a synchronous motor using permanent magnets, a vector control system is practically applied for driving the AC motor of the electric vehicle, whereby current of the motor is separated into a torque current Iq and an exciting current Id. In a system disclosed in Japanese Patent Application Laid-Open No. 7-212915, only a q-axis current command Iq* is compensated based on a voltage and a current of the motor and a signal of a temperature sensor in order to compensate variation of output power caused by temperature rise of the magnets of the permanent magnet synchronous motor.
The output power or the torque of the permanent magnet type synchronous motor is decreased by a decrease of the induced o voltage caused by temperature rise of the magnets. Characteristic of the output power decrease caused by the temperature rise of the magnets of the permanent magnet type synchronous motor differs depending on a material of the magnets. For example, the characteristic of output power decrease for a ferrite magnet is approximately -0.2%/.degree. C., and that for a neodymium magnet is -0.1%/.degree. C. Particularly in a compact-sized synchronous machine which is sensitive to temperature rise of the permanent magnets due to its small size the magnitude of decrease in the output power or the torque caused by the temperature rise is substantially large.